Processes for gas pressure sintering for the production of ceramic molded parts from Si.sub.3 N.sub.4 powders have gained increasingly in importance. Gas pressure sintering has the advantage over the hot press process that it can be used for producing shapes of any degree of complexicity and it has the further advantage of costing much less than the hot isostatic pressure molding process.
Si.sub.3 N.sub.4 powders, however, cannot be sintered in their pure form. It is only after so called sintering additives have been added to the material is capable of sintering and hence of completely densifying. Oxidic materials such as MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 have, in many cases, proved to be useful sintering additives but nonoxidic substances such as AlN, Mg.sub.3 N.sub.2, and soforth have also been found satisfactory. In the process of sintering, the sintering additives form liquid phases together with a proportion of the ceramic material and thereby initiate the mechanism of liquid phase sintering. After the sintering process, the sintering additives are present in the structure as secondary phases.
The structure of the sintered body and hence its strength, one the most important properties of ceramic components, is determined to a large extent by the uniformity with which the secondary phase is distributed in the structure before, during and after sintering. If the liquid phase is unevenly distributed during the sintering process then the whole structure will be inhomogeneous after sintering. The statistical reliability of the prediction of expected strength values of the sintered ceramics material is thereby reduced. Incomplete adjustment of equilibrium in the formation of the liquid phase due to uneven distribution has particularly adverse effects if the liquid phase is thereby prevented from recrystallizing on cooling, as it would be expected to do if equilibrium were completely adjusted, and instead solidifies as a glass-like substance in the structure. Glassy secondary phases soften at much lower temperatures than crystalline secondary phases and thereby drastically reduce the strength at high temperature. Homogeneous formation of the liquid phase conforming to the chemical equilibrium is therefore an important criterion for gas pressure sintering of Si.sub.3 N.sub.4.
If a homogeneous liquid phase is to be obtained, the sintering additives should as far as possible already be uniformly distributed in the material before sintering takes place. For this purpose, the Si.sub.3 N.sub.4 powders and pulverulent sintering additives are generally ground together in wet mills (attrition mills, ball mills, planet mills, etc). This not only breaks down any agglomerations of powder but also ensures a statistically uniform distribution of the sintering additives throughout the starting material.
Si.sub.3 N.sub.4 powders prepared by wet grinding, however, still have serious disadvantages due to abrasion of the grinding bodies and to the fact that the powders reagglomerate on drying so that relatively hard agglomerates are again formed.
It is therefore an object of the present invention to provide a sinterable Si.sub.3 N.sub.4 powder which does not have these disadvantages.
Our co-pending application Ser. No. 54,029, filed May 22, 1987, discloses and claims silicon nitride powders having an average particle size of less than 1 micron and no particles with a diameter greater than 100 micron, said powder having a total metallic impurity content of less than 1000 parts per million and an iron content of less than 200 parts per million.